Investigation Of Plasma Sprayed NiCr-Cr₃C₂ Coating On Mould CrZrCu Alloy Plates

The Cr₃C₂-NiCr coating,NiAl coating and NiAl/Cr₃C₂-NiCr coating were prepared by plasma spraying on the CuCrZr alloy for mould. Firstly, the plasma spraying processing parameters were optimized by the orthogonal test with the bonding strength between the coating and the substrate as an indicator. And then, the thermal shock test of Cr₃C₂-NiCr coating and NiAl/Cr₃C₂-NiCr coating which prepared with the optimized parameters was done to investigate the thermal shock resistance of the coatings. The hardness of the Cr₃C₂-NiCr coating and NiAl coating were tested to analyze their distribution. And also microstructure and fracture of the plasma sprayed coating were investigated by means of SEM, EDS, XRD and so on. So, the effects of four parameters, including spraying distance, primary gas flow rate, deliver powder rate and spray power, on the plasma sprayed Cr₃C₂-NiCr coating, NiAl coating and NiAl/Cr₃C₂-NiCr coating were studied. At last, the feasibility of improving the bonding strength was studied through vacuum diffusion heat treatment for Cr₃C₂-NiCr coating. Within the processing parameters of this experiment, the results are as follows.For Cr₃C₂-NiCr coating, the key process factors on the bonding strength are ranked as the sequence: power, powder feeding rate, spray distance and primary gas flow. The bonding strength of the coatings which were prepared by plasma spraying with the optimized process parameters is 60.82MPa.For NiAl coating, and the effects of process factors on bonding strength are ranked as following: power>powder feeding rate>spray distance > primary gas flow. After optimization, the bonding strength reaches into 48.99MPa. For NiAl/Cr₃C₂-NiCr coating, the relationships between the effects of process factors on bonding strength are ranked as: power>primary gas flow>spraying distance>powder feeding rate. The bonding strength is 50.60MPa after process parameters optimized.Cr₃C₂-NiCr coating and NiAl/Cr₃C₂-NiCr coating have good thermal shock resistance and high thermal fatigue life which reaches higher number of macro-crack initiation and failure times. Thermal shock resistance of NiAl/Cr₃C₂-NiCr coating is better than that of Cr₃C₂-NiCr coating. Its number of macro-crack initiation and failure times is also more than Cr₃C₂-NiCr coatings'under the same conditions. The coating thermal shock failure is due to the high mismatch thermal stress in the interface which is caused by the difference in coefficient of thermal expansion between coating and substrate. Forms stress concentration and the thermal fatigue cracks in the interface from the angle of the specimen, crack propagation cause the coating interfacial debonding. In addition, the oxidation of the interface promotes thermal shock failure. The main failure molds for the coating are the cracks of the coating initiate, expand and then overall peel from the interface along the coating and substrate, and the failure of small pieces off, moreover, failure molds are associated with temperature.The coatings are well bonded and have low porosity. There is exothermic reaction between Ni and Al during the process of plasma spraying the NiAl coating,and various kinds of intermetallics such as Al₄Ni₃,Al₃Ni₂,AlNi₃ and so on are formed at the same time. The remaining Al can react with Cu substrate to form Cu₃Al₂,CuAl₂ and CuAl,thus there is a micro-metallurgical bonding in some micro-zone between the coating and substrate, but the bonding was mainly mechanical bonded.Both in the parallel and perpendicular directions compared with substrate, the micro-hardness of Cr₃C₂-NiCr coating and NiAl coating has greatly dispersion. But in the parallel direction to substrate, they significantly conform to normal and weibull distribution.Through the vacuum diffusion heat treatment, solution treatment and then aging treatment,not only hardness and electrical conductivity can be completely recovered to meet the CuCrZr alloys'performance which are called for mould, but also can significantly increase bonding strength of coating. Compared with the non-heat treatment coating, the bonding strength of experimental coating increased 4.43 times.